Elcometer 122 Testex Tape
Elcometer 124 Dial Thickness Gauge
Test Method

In summary, ASTM D 4417 Method C and NACE RP0287-95 require the appropriate tape to be rubbed onto the surface.

The thickness of the tape, when the thickness of the backing has been subtracted, gives the peak-to-valley height of the surface.

Measurements should be made at an agreed series of locations, which characterise the surface.